I am not sure if I know what you are talking about but Poston's has all of the parts for the shifter and really cheap too. I haven't used any of this stuff so don't know the quality., but the price is right.
Do you mean the little white tip inside the chrome part of the shifter? If so, how is it broken?
If your shifters not making you use the button, have you made sure the spring is still in place under the shifter plate?
You need to check the spring as someone mentioned and also check the J-shaped metal piece at the end of the shifter rod...if it gets bent it wil cause the white plastic piece to jam...remove the shifter plate then remove the black plastic piece for the gear indicator then remove the ribbed piece with the hole that goes over the shifter rod ...you should then be able to see the bottom of the shifter...check to see of the spring is in
place...check the J-shaped piece...just had this happen...too many times slammming the shifter into gear with the transbrake...I took some pliers and straightened the "J" piece...works OK now...